1. Field of the Invention
The present invention relates to porous ceramics provided with amorphous pore surfaces useful for treating various kinds of drain water and waste liquid which treatment now becomes a serious problem, such as river or lake water and industrial drain water containing hard to remove phosphorus and nitrogen, waste water containing organochlorine compounds such as tetrachloroethylene, piggery excreta waste water containing organic nitrogen at high levels, waste water containing heavy metals such as lead, hexavalent chromium, nickel, mercury, zinc, cadmium, selenium, etc., drain water from dairy factories, aquatic food processing factories and slaughter houses which contains water soluble protein at high level, drain water from pulp industry, photographic developer waste water, car wash drain water containing a mixture of polishing wax and detergent and the like, and a method for preparing the same and a method for treating various kinds of drain and waste water by the use of the porous ceramics and activated carbon.
2. Description of the Related Art
Recently, one serious social problem is pollution of water areas, for example, lake eutrification caused by an excessive supply of nutritive salt such as phosphorus, nitrogen, etc. from river basins thereof, and further, an inflow of similar nutritive salt into basins, etc. due to concentration of both population and industry, which results in abnormal breeding of algae and an occurrence of red tide thereby damaging fisheries.
As a lake is a closed water basin where water remains therein long enough to easily precipitate contaminants, water pollution thereof is more serious than that of rivers and sea areas and causes difficulty in filtering tap water, problems of off-flavor or -taste and troubles of a water use in the field of fisheries, etc.
Conventional methods for treating drain water and waste liquid containing phosphorus and nitrogen are generally known as in the following. As to phosphorus, the most general is a coagulation method using a coagulant, which comprises, for example, an anaerobic-aerobic primary treatment, a coagulating secondary treatment using a coagulant and a reverse osmotic and ion exchanging final treatment. Although an absorbing or crystallizing treatment is also known, the coagulation method is often used for industrial drain water, while the anaerobic-aerobic method is used for sewage, in general. As to nitrogen, various kinds of treating methods are known such as nitrification-denitrification, ammonia stripping, selective ion exchange, break-point chlorination, catalytic oxidation and the like, although the anaerobic-aerobic treatment (nitrification-denitrification) and a combined method of the coagulation and anaerobic-aerobic treatment are practically used among others.
Tetrachloroethylene is an odorous liquid organochlorine compound having higher specific gravity and used as a solvent for laundry, metal cleaning, etc., but as is well known, exhibits adverse effects in humans and causes headache, giddiness, hepatic dysfunction and the like. Trichloroethylene is also an odorous liquid organochlorine compound having higher specific gravity and mainly used as a metal cleaning solvent because of adequate compatibility thereof with oil, but as is well known, causes headache, giddiness, hepatic dysfunction and the like.
In a conventional method for generally treating waste liquid and drain water containing organochlorine compounds such as tetrachloroethylene, trichloroethylene, these toxic substances were recovered by means of an activated carbon unit or a moisture separating unit as a pretreatment to reduce concentration thereof in the waste liquid and drain water, followed by an aeration treatment.
Heretofore, it has been known to treat various kinds of waste water by the use of porous ceramics.
JP-A No. 58-205,516, for example, describes a superior tubular cartridge-type ceramic filter for selectively removing chemicals, heavy metals, etc. in which a silicastone-clay mixture is added with feldspar and alumina powder, kneaded with saw dust and water to form a muddy molded article and calcined at a temperature of 1,100 to 1,200xc2x0 C.
JP-B No. 1-60,317 describes a waste water treating system for sewage, etc. which comprises a filter layer consisting of sand and gravel layers formed on the bottom of a treating tank, a treating layer formed by laminating porous ceramic particles on the filter layer and air diffuser tubes arranged between an upper surface of the treating layer and a bottom surface of the filter layer.
JP-A No. 61-136,490 describes an aeration-type waste water treating system in which filter materials comprising porous ceramic particles filled in a perforated material such as a net are vertically arranged at certain spaces on a location so as to cross the direction of a waste water flow in an aeration tank.
JP-B No. 1-42,758 describes an advanced aeration-type waste water treating system which comprises an aeration tank in which filter materials comprising porous ceramic particles filled in a perforated material such as a net are vertically arranged at certain spaces on a location so as to cross the direction of a waste water flow in the aeration tank, a filter layer connected with the aeration tank through a tube body and consisting of sand and gravel layers on a bottom portion thereof, a treating layer formed by laminating porous ceramic particle on the filter layer and diffuser tubes arranged between an upper surface of the treating layer and a bottom surface of the filter layer.
JP-B No. 2-1,558 describes a method for biologically treating waste water such as domestic, general industrial or food processing waste water in the main in a treating medium containing porous ceramics and wood chips as main components.
JP-B No. 63-66,247 describes a method for treating emulsifiable oil containing waste water in which a mixture of porous ceramics and activated carbon is filled as a filter material in a cylindrical body to use as a filter system.
JP-A No. 60-261,586 describes a method for removing metals, metal ions and organochlorine compounds in drinking water such as tap water by means of a filter system with an intermediate layer of activated carbon between porous ceramics layers.
Further, as a method for preparing porous ceramics useful for the treating the above mentioned drain and waste water, JP-A-No. 61-291,473 describes a method for preparing porous ceramics by compounding a pore forming material to a slurry of clay mineral material added by and mixed with water followed by drying and calcination, characterized in that the pore forming material is subjected to a pretreatment to combine a metal powder or organometalic compound to a pore surface, while the calcination is conducted in an atmosphere of air or nitrogen.
However, when the above mentioned methods or conventionally known porous ceramics are used to treat various kinds of drain water and waste water containing, for example, phosphorus, nitrogen and organochlorine compounds such as tetrachloroethylene and trichloroethane which treatment is now becoming a problem, it has been impossible to completely remove these toxic contaminants therefrom.
In addition, according to an example described in JP-A No. 61-291,473 as mentioned above, a dry molded product is heated up and calcined at a temperature of about 1,200 to 1,500xc2x0 C. for an hour to form porous ceramics. The thus formed ceramics are generally homogeneous in quality because of such heating up to 1,200 to 1,500xc2x0 C. in rather a short period of time by means of an electric kiln and calcination for an hour or so, which does not change a pore surface thereof to amorphous but results in scattering compressive strength. As ceramics of less compressive strength are easily pulverized, clogging of drain water or waste water treating columns occurs when these ceramics are filled therein, which makes it difficult to use such columns longer, while they exhibit not enough ability to remove toxic contaminants nor long-term durability when they are used to treat various kinds of drain water and waste water. That is, such ceramics can not be used practically.
The present inventors have studied to develop practical and long-term durable ceramics which can remove toxic contaminants such as phosphorous (P); nitrogen (N); organochlorine compounds, e.g., tetrachloroethylene, etc.; and heavy metals, e.g., hexavalent chromium, lead, etc. from drain water and waste water containing these contaminants to a level lower than the Japanese environmental water quality standard or to a level of almost zero. While reviewing in detail conventional information relating to porous ceramics from a viewpoint completely different from existing knowledge in how to prepare homogeneous porous ceramics has been studied, porous ceramics provided with amorphous pore surfaces were prepared, and it was unexpectedly found that the thus prepared porous ceramics are effective and practical for treating various kinds of drain and waste water. Thus, the present invention has been accomplished.
The present invention relates to a method for preparing porous ceramics provided with amorphous pore surfaces and a method for treating drain water and waste liquid containing phosphorus, nitrogen, organochlorine compounds such as tetrachloroethylene, etc. by the use of same. Further, the present invention relates to a method for preparing porous ceramics provided with amorphous pore surfaces by mixing clay, a pore forming material and water, molding into an adequate shape, followed by drying, heating up the temperature of molded articles thus dried from ordinary temperature to 600 to 800xc2x0 C. over a time period of 5 to 15 hours, desirably 10 hours or so by, for example, self-burning of the pore forming material mixed in the articles, then maintaining temperature thereof at 600 to 800xc2x0 C. for 3 to 7 hours, desirably for about 5 hours, consecutively heating up to 1,200 to 1,500xc2x0 C., calcining the molded articles at the heated up temperature for 4 to 8 hours, desirably for about 6 hours, and subjecting a crushing treatment after cooled off, and a method for treating drain water and waste liquid containing phosphorus, nitrogen, organochlorine compounds such as tetrachloroethylene, etc. by the use of same.
In addition, the present invention relates to a method for treating drain water or waste water containing phosphorus, nitrogen, organochlorine compounds, etc. by the use of a mixture of porous ceramics provided with amorphous pore surfaces and activated carbon, desirably the mixture comprising the same weight of such components.
Further, the present invention relates to a method for treating drain water and waste liquid containing phosphorus and nitrogen in which the drain water and waste liquid is introduced to a top opened storage-open type aeration tank containing a mixture of porous ceramics provided with amorphous pore surfaces and activated carbon, subjected to aeration from the bottom of the tank and then passed through a column filled with the mixture of porous ceramics with amorphous pore surfaces and activated carbon, a coagulation treatment using a coagulant being used together with either of above mentioned steps, if necessary.
Furthermore, the present invention relates to a treating system for drain water and waste liquid containing phosphorus and nitrogen in which a coagulation-precipitation tank arranged therein at need, a top opened and bottom aerating storage-open type aeration tank containing porous ceramics provided with amorphous pore surfaces and activated carbon and a column filled with a treating agent containing porous ceramics provided with amorphous pore surfaces are arranged.
The present invention also relates to a method for treating drain water and waste liquid containing tetrachloroethylene, Trichloroethylene, trichloroethane, dichloromethane, carbon tetrachloride and the like, and a method for treating drain water and waste liquid containing organochlorine compounds such as dry cleaning waste liquid, etc.
The present invention also relates to a method for treating waste liquid and drain water containing organochlorine compounds in which the waste liquid and drain water containing organochlorine compounds is passed through and treated in a column filled with a treating agent containing a mixture of porous ceramics provided with amorphous pore surfaces and activated carbon, desirably the mixture comprising each component at a mixing ratio by weight of 50 to 50.
The present invention also relates to a treating system for waste liquid and drain water containing organochlorine compounds which comprises a column filled with a treating agent containing porous ceramics provided with amorphous pore surfaces, desirably a mixture thereof with activated carbon mixed at an equal weight ratio, and a means to pass the waste liquid and drain water containing organochlorine compounds to the column.
The present invention will be further described in detail in the following.
Clay used to prepare porous ceramics provided with amorphous pore surfaces of the present invention includes clay mineral which is adhesive and plastic in quality when water is added, hardened when dried and contains silicate mineral as a main component thereof and, if necessary, may further contain inorganic components such as calcium, magnesium, sodium, potassium and the like.
A pore forming material used to prepare porous ceramics provided with amorphous surface of the present invention includes any material originated from a plant sawdust, wood chips, chaff, straw, wheat bran, bark and the like which generates gas when the material is heated, plastic dust, particulate absorbent high polymer, sodium carbonate and the like. Sawdust and wood chips are desirably used because these materials form porous ceramic structure with amorphous pore surfaces without radical generation of gas at a self burning step thereof.
Tap water is generally used to prepare porous ceramics provided with amorphous pore surfaces generally includes tap water, while sea water may also used as xe2x80x9cwaterxe2x80x9d when the clay mineral contains less alkaline components. If clay or a pore forming material as the starting material contains water, such water may be used as moisture.
In a method for preparing porous ceramics provided with amorphous surface of the present invention, kneading of clay, a pore forming material and water may be carried out to homogenize them in arbitrary order by means of a kneading machine such as a mortal cement mixer, etc. Although any mixing ratio of these materials may be used if porous ceramics provided with amorphous surface of the present invention can be obtained, a mixture of 2 parts by weight of clay, 3.2 parts by weight of water and 6.76 parts by weight of the pore forming material is desirably, but unrestrictedly used so that many amorphous areas are formed on continuous pore portions to absorb substances to be treated on residual unsmooth portions.
The thus kneaded materials are then molded into an arbitrary shape convenient for calcination, such as a brick shape, a disk shape and so on.
In a method for preparing porous ceramics provided with amorphous surface of the present invention, drying of the thus molded product is usually but unrestrictedly conducted by leaving the product intact at room temperature to attain a water content of 40%, and may also be carried out through blowing by means of a burner, etc.
In a method for preparing porous ceramics provided with amorphous surface of the present invention, heat-up and calcination may be conducted by any kind of means which can heat up and calcine the molded articles as described above after drying, such as an electric kiln, a ceramics kiln made of fire bricks and the other type of kiln, these kilns being desirably used from a viewpoint of mass production of uniform ceramics at a time.
In a method for preparing porous ceramics provided with amorphous surface of the present invention, the molded articles after calcination may be directly used as filter media of a block type and a cartridge type, while such articles are usually subjected to a crushing treatment when they are used to fill a column, etc. Although any type of roller crushers may be used to conduct the crushing treatment, it is desirable to use a roller crusher improved by the present inventors to leave the narrowest space of 10 to 30 mm between rollers so as to reduce the formation of powdery ceramics unsuitable to use for a column, etc.
In the following, there will be detailed the most important processes of heat-up and calcination in a method for preparing porous ceramics provided with amorphous surface of the present invention.
First of all, temperature of molded and dried articles is gradually heated up from normal temperature to 600 to 800xc2x0 C. and desirably to 800xc2x0 C. An adoption of such a slow heat-up process allows properties of sintered ceramics to maintain about the same form as that of a molded point of time, thereby forming sufficient continuous pores and yielding reaction sites for substances to be treated.
On the other hand, sintered ceramics might be brittle because of an insufficient linkage between crystalline components, when a rapid heat-up process is adopted by means of an electric kiln, etc. according to a conventional method in which the slow heat-up is not used, such as, for example, described in a method for preparing porous ceramics of the above mentioned JP-A No. 61-291,473. At the same time, most of the pore surfaces formed on the ceramics are covered with crystalline components to increase passing resistance of components to be treated through the pores, thereby yielding ceramics in which the components to be treated permeate insufficiently into the pores. When such ceramics are used to remove contaminants and toxic components in drain water and waste liquid, removal thereof proceeds unsatisfactorily, while it is impossible to obtain long term durable ceramics.
A time period required to conduct the above mentioned slow heat-up process from normal temperature to 600 to 800xc2x0 C. usually ranges 5 to 10 hours and is desirably about 10 hours, depending on a size and an amount of the molded articles. While the heat-up process can be controlled by adjusting temperature of an electric kiln and a ceramics kiln or the other type of kiln to be heated or warmed, it is especially desirable to burn a corner of the molded and dried articles by means of, for example, a burner when the pore forming material is a burnable material such as sawdust, the heat-up process being conducted by burning of the burnable pore forming material per se, i.e., self burning.
The method for controlling temperature of the molded articles due to self burning has been epochally found by the present inventors on the way to prepare the porous ceramics and, as a result, it is confirmed that the above mentioned method can provide porous ceramics provided with amorphous surface useful for treating drain water and waste liquid containing phosphorus, nitrogen, organochlorine compounds and the like.
As has been described above, the burnable material such as sawdust, etc. in the molded articles is ashed at a stage when temperature of the molded and dried articles heated up to 600 to 800xc2x0 C. over a time period of 5 to 15 hours. At this stage, the temperature ranging from 600 to 800xc2x0 C. is kept by additional burning for 3 to 7 hours, desirably for about 5 hours. This process is essential for preparing porous ceramics provided with amorphous surface of the present invention.
An adoption of this process allows alkaline components in clay to melt out gradually, thereby changing continuous pore surfaces of porous ceramics to amorphous and improving strength of sintered porous ceramics. When conventional heat-up is conducted rapidly from normal temperature to 1,200 to 1,500xc2x0 C. without adopting the above mentioned process, quality of sintered ceramics varies widely and in addition, ability thereof to remove toxic components and contaminants becomes insufficient.
While compressive strength of thus prepared ceramics varies widely, for example, ceramics of lower compressive strength are powdered during a crushing process after calcination thereof, which easily cause clogging because of considerable fluid resistance and, as a result, can not be practically used when such powdery ceramics are filled in a column to treat drain water and waste liquid.
The molded articles heated to 600 to 800xc2x0 C. for 3 to 7 hours are then heated up to 1,200 to 1,500xc2x0 C. over a time period of about 4 hours.
Due to such a relatively wider range of calcination temperature from 1,200 to 1,500xc2x0 C. in the present invention, amounts of silicon dioxide and alkaline components in clay are controlled to result in difference in active surfaces as a result of calcination. The calcination temperature is about 1,200xc2x0 C. when an alkaline content in the clay is relatively rich and calcination is conducted easily, while the temperature is 1,500xc2x0 C. when a silicon dioxide content in the clay is rich and calcination is conducted hardly. Accordingly, calcination temperature of about 1,250xc2x0 C. is especially preferable to obtain ceramics to be used for various purposes.
When temperature of the molded articles goes up to 1,200 to 1,500xc2x0 C. calcination is conducted at this temperature for 4 to 8 hours, desirably for about 6 hours. If a time period of calcination at this temperature is only an hour or so as described in JP-A No. 61-291,473, brittleness is a typical disadvantage of ceramics thus obtained, because surfaces of continuous pore forming portions is insufficiently changed to an amorphous state, thereby yielding less sintering strength with surrounding particles.
The term xe2x80x9camorphous pore surfacexe2x80x9d used herein means that surfaces of continuous pores formed inside of ceramics comprise a combination of an amorphous material containing sodium silicate as a main component and a crystalline material (crystalline particles) such as sodium silicate, calcium oxide etc. The crystalline portion (crystalline particles) reacts with components to be treated in waste water, while the amorphous portion makes little contribution to the reaction.
As a result of electron microscopy of continuous pore surfaces using HITACHI emission-type electron microscope S-4200 model under a condition of accelerating voltage of 10 KV (secondary electronic image) and 20 KV (reflective electronic image) and a magnification of 3,000, it is observed that not only ceramic particle surfaces 1 but most of surfaces of continuous pores 2 of conventional ceramics such as, for example, porous ceramics described in JP-A No. 61-291,473 are covered by crystalline particles 3 as shown in FIG. 1. Even when a flow 4 of components to be treated reaches the ceramic particles and is ready to pass through the continuous pores 2, these components can not easily enter into or permeate through the pores 2 because of resistance of the ceramics. Almost exclusively, the ceramic article surfaces 1 make contribution to the reaction with components to be treated in this case. By contrast, as shown in FIG. 2, there exist flat and amorphous portions 5 on surfaces of continuous pores 2 of porous ceramics provided with amorphous surface of the present invention. When a flow 4 of components to be treated reaches the ceramic particles and passes through the continuous pores 2, these components can easily enter into or permeate through the pores 2 with less resistance of the ceramics. Accordingly, not only the ceramic particle surfaces 1 but the crystalline particles 3 in the pores 2 reacts with the components to be treated, thereby considerably improving a treating effect of drain water and waste liquid containing phosphorus and nitrogen.
Activated carbon used in the present invention includes any product prepared by thoroughly carbonizing a raw material such as charcoal, coconut shell and coal-char as well as animal bones, blood and the like, and is not limited to a certain commercially available product if the material is carbonaceous and has large relative surface area and high absorbing ability.
A coagulant used in the present invention includes any agent which is not limited especially but can coagulate fine suspending contents of drain water to form flocs, while, for example, an inorganic coagulant such as iron trichloride, ammonium sulfate, calcium hydroxide, etc., and an organic high-molecular coagulant comprising a surfactant or a high molecular coagulant may be simultaneously used.
A storage-open type aeration tank used in the present invention includes any conventional open type aeration tank and, for example, a top opened cylindrical tank of 50 cm in diameter and 60 cm in height in which an air introducing pipe is connected to the bottom thereof, while air introduced in the pipe by a compressor or a blower is homogeneously dispersed through a porous panel or a diffuser arranged on the bottom of the tank to perform aeration. Liquid to be treated is stored in the tank during such aeration.
Columns used in the present invention includes any conventional column used for an absorption treatment and, for example, a column system in which three columns are arranged vertically and connected in series. A column arrangement may be varied arbitrarily so far as effects of the present invention are obtained, and accordingly such columns may be aligned in an inclined condition, connected in series to form a long type or arranged in parallel. Liquid is desirably passed through the columns in the direction from bottom to upward.
It is confirmed that porous ceramics provided with amorphous pore surfaces of the present invention is useful for treating various kinds of drain water and waste liquid which treatment now becomes a problem, for example, drain water and waste liquid containing hardly removable phosphorus and nitrogen, waste liquid containing organochlorine compounds such as tetrachloroethylene, etc., excretive drain water from a piggery containing organonitrogen compounds at a high level, waste liquid containing heavy metals such as lead, hexavalent chromium, nickel, mercury, zinc, cadmium, selenium and the like, drain water from dairy product plants, fishery processing plants, slaughterhouses, etc. which contains water soluble protein at a height level, drain water from pulp plants, photo developing waste liquid, car wash drain water containing car polishing wax and detergent and the like.
The above mentioned drain water and waste liquid containing phosphorus and nitrogen includes drain water from rivers and lakes and drain water and waste liquid from various kinds of plants such as food industry including, for example, dairy product, fishery product, fermented food product, etc.; cattle industry; pulp and textile industry; printing industry; various chemical industries; petroleum refining industry; glass and sodium industry; steel industry; metal and mechanical surface treatment industry, etc.; as well as sanitary sewage from reclaimed sites of incombustible material and drain water from hospitals, laundries, laboratories and the like.
The above mentioned drain water and waste liquid containing organochlorine compounds such as tetrachloroethylene, trichloroethylene, dichloromethane, carbon tetrachloride, 1,2-dichloroethylene, 1,1-dichloroethylene, cis-1,2-dichloroethylene, 1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,3-dichloropropene and the like includes, other than drain water from laundries including dry cleaning, waste liquid and drain water containing organochlorine compounds from chemical industry, metal product manufacturing industry, electrical appliance manufacturing industry, transportation equipment manufacturing industry, precision equipment manufacturing industry, textile industry and the like.